1. Field of the Invention
This invention relates to a two steps process for the destruction of PCB by high temperature treatment of the halogenated structure in the presence of an alkali or alkaline earth metal generated "in situ", followed by oxidation of resulting products.
2. Prior Art
Polychlorinated biphenyls (PCB) and other polyhalogenated structures such as lindane or dioxin are known as highly toxic substances. Because of their inertness, the destruction of such materials is not easily achieved. The level of residual polyhalogenated material, after treatment, must be of the order of ppm or lower to satisfy the requirement of the environmental regulating agencies.
Several methods have been reported for the disposal of PCBs. Generally, three broad methods have been disclosed in the literature: combustion, catalytic oxidation and heating to a temperature of less than 150.degree. C. in the presence of elemental sodium.
The simplest approach proposed has been the combustion of the material to be disposed of under a variety of conditions. For examples, Suggitt (U.S. Pat. No. 4,468,376) describes partial oxidation of the PCB in the presence of hydrocarbonaceous material, Rathjen et al (U.S. Pat. No. 4,140,066) reports the combustion of similar products in a specially designed combustion chamber, Robinson (U.S. Pat. No. 4,198,384) recommends partial oxidation followed by quenching of the reaction products and Meenan et al (U.S. Pat. No. 4,402,274), Winnen (U.S. Pat. No. 4,018,879) and Scheifley et al (U.S. Pat. No. 4,125,593) describe combustion processes with heat recovery.
Other authors have reported the use of different catalyst to promote the degradation of PCB, under oxidation conditions. For example, a Pt catalyst is used by Rasp et al, in U.S. Pat. No. 4,151,262, Kageyama in U.S. Pat. No. 4,053,557, and Kageyama in U.S. Pat. No. 3,972,979. A Cr catalyst is used in Johnson, U.S. Pat. No. 3,989,807 and Fe, Cu or Mn catalysts are used in Hyatt, U.S. Pat. No. 3,989,806. In some instances, reductive catalytic operations are indicated: Schafe (U.S. Pat. No. 3,892,818), Oricchio (U.S. Pat. No. 3,855,347) Brainerd et al (U.S. Pat. No. 2,803,669) or Wu-Chi Chen (U.S. Pat. No. 4,397,829).
Low temperature or room temperature processes have been reported calling upon alkali metal aromatic radical anion or sodium alcoholates as active reagents to degrade PCB in an inert atmosphere. Such approaches are described by Pytlewski et al (U.S. Pat. No. 4,417,977), Parker et al (U.S. Pat. No. 4,447,667) and Pytlewski et al (U.S. Pat. No. 4,430,208).
Oxidations of PCB in the presence of molten salts that are inert towards the reaction products are described by de Benckelaer (U.S. Pat. No. 3,969,490) and Grantham (U.S. Pat. No. 4,246,255).
Finally, dispersions of sodium are reported to react with PCB at low temperature by Norman et al (U.S. Pat. No. 4,379,746), Norman (U.S. Pat. No. 4,379,752) and Jordan (U.S. Pat. No. 4,340,471).
It has been found that each of the methods proposed by the prior art have different drawbacks. For example, generally speaking these methods either do not completely destroy the PCB or when they do the cost of operation is very high and requires in many cases the use of complex equipment with the handling of high volumes of materials derived from the breakdown of PCBs.
The reactions between PCB and an alkali such as sodium can be made essentially complete but the high cost of sodium or sodium alcoholate and the sensitivity of such reagents towards moisture which quite generally contaminates PCB thereby render their use less attractive.
On the other hand, the oxidation processes, whether direct or catalytic, suggested by the prior art, are not attractive because of the large investments required to insure the complete destruction of PCB.
It is also obvious that most of the methods proposed are deficient in safety and efficiency since large amounts of PCBs and other polyhalogenated materials are kept in storage at a very high cost, pending destruction.
Accordingly, it would appear highly desirable if a method for the destruction of PCBs and other polyhalogenated aromatics could be provided and which would combine efficiency and low cost of equipment and operation.